1. Field of the Invention
The present invention relates to a structure that may be formed of a plurality of resinous cells, a method and an apparatus for forming such a structure. The structure may be formed in place where it is to be utilized.
2. Description of Related Art
In many situations, it may be desirable to form a structure in place where it is to be utilized. Sometimes, a structure is preformed and then assembled in place. One proposed application of such structures is in outer space. For example, it has been proposed to form spacecraft or portions of spacecraft from prefabricated inflatable structures. Prefabricated inflatable structures have also been proposed for forming structures on other celestial bodies, such as moon bases and underground caverns. Such structures have also been proposed for use in terrestrial applications.
Space-based structures that are assembled and/or formed in place can include a plurality of layers of material with various functions. In the context of a spacecraft, the structures can include layers for meteorite resistance, gas retention for inflatable structures, layers for helping the structure to maintain its shape, layers to help retain heat, and/or layers for other functions.
Space based structures have been proposed that are prefabricated and then inflated in space. For example, satellites have been proposed and deployed, which include an inflatable structure. According to one example, the satellite referred to as Explorer XIX, which was launched in December 1963, included a 20 inch inflatable structure. The structure was inflated with gas and included 40 gore plies of preform over a hemispherical mold. The skin was a four-ply laminate consisting of alternating layer so aluminum foil and mylar film, each 0.5 or 0.013 mm thick. The inflatable structure maintained its shape after evacuation of the inflating gas.
Other satellites formed from inflating structures include the ECHO satellites, such as the PAGEOS satellite launched in June of 1966. This satellite included 84 gore plies sealed with adhesive tape. The structure was inflated utilizing a mixture of subliming powders. During ground-based tests, the inflated structure was found to differ from the designed dimensions by less than one-half of one percent. This accuracy was obtained at least in part by utilizing an accurate gore ply pattern, by maintaining seal tolerances and by lowering sealing temperature.
Another proposed use of inflatable structures is as a protective expandable enclosure for astronauts. In particular, an enclosure was built that included a composite wall having an inner three-barrier pressure bladder for gas retention, a four-ply Dacron cloth structural layer, a 2-inch thick polyether foam meteoroid barrier, and a film-cloth laminate outer cover with thermal coating. The expandable composite wall could be structurally bonded to a rigid aluminum honeycomb sandwich floor.
Another example includes an inflatable antenna. The antenna as it has been proposed includes a large, pressurized antennae. In a structure shaped by internal pressure, using a low modulus plastic film, the pressure level decreases with the third power of the linear dimension. As a result, lost inflatant mass would decrease with increasing antenna size. This would make it possible to build low-mass inflatable systems with 5- to 10-year lifetimes. One design incorporates a stabilization torus, with a pressure sufficiently high to warrant the use of a rigidizing structure.
Another proposal includes an inflatable, space-rigidized structure (ISRS). One example of this structure is a 10-m antenna reflector. The antenna consisted of a thin, fiber-reinforced composite lamina. The materials used were a lightweight KEVLAR cloth in an imide-modified, catalytically cured, cycloaliphatic epoxy resin, which may be cured either thermally or by an external gaseous catalyst. The area mass of the resulting composite wall, including a plastic foil gas barrier, was of the order of 0.1 kg/mz.
According to another example, the SOLARES was a 1000 m diameter flat light-reflective membrane stretched by an inflatable toroidal hoop stabilized by tension lines to two masts normal to the membrane at the center. Also, an inflatable antenna has been created that includes a large microwave or light collecting or reflecting dish fabricated using an inflated torus for the rim and either parabolically shaped or spherical shaped plastic membranes attached to the torus. The membranes were stretched to shape by air pressure. The concave surface was metallized to produce the desired microwave reflectivity.
It has been proposed to form a structure by blowing large bubbles in space using self-rigidizing liquids. This would allow the fabrication of large flat structures. Then, self-rigidizing foam could be used to inflate and subsequently rigidize inflatable structures.
As apparent from the above discussion, a number of methods for fabrication of structures in space have been postulated. A number of the proposals have involved inflated structures, clever mechanisms that unfold and lock into position, or polymeric based component systems that can be processed real time. Each has benefits and drawbacks.
Attempts to inflate a structure have been successful in initial deployment, but require a method of maintaining the pressure or stabilizing the structural elements once the gas has been used to position them. Long-term gas supply requires dedicated mission payload capacity. The gas must be maintained in a leak free containment or the structure will collapse. Any gas that does leak or diffuse through the structure over time will be a possible contaminant to sensors and solar cell arrays. This is true even for a structure that only depends on the gas for deployment, not to maintain structural rigidity.
A mechanical approach has been successful in a number of cases, but it is limited to smaller structures due to the inefficiencies inherent in joints, fasteners and sliding surfaces. A number of deployment failures have occurred that were traced to environmental effects, such as thermal expansion and contraction or to tribological effects.
It has also been proposed to fabricate polymeric-based composite structures in space, by pultruding or extruding the materials through a heated processing device. However, the energy requirements for heating can be very taxing on a spacecraft's power system. The residual stresses developed in a composite structure as it is heated during curing and cooled to ambient, and possibly very low, temperatures add complication to the design. Another detrimental factor of such techniques is outgassing of the resin as it is processed. Again, this presents a contamination issue. One approach to get around this issue has been the exploration of using ultraviolet (UV) curing resin systems. The primary drawback to this technology has been the inability to develop a process that allows the resin to be cured with UV while the composite material is being compacted. A composite structure cured with no compacting load will have a substantially reduced performance due to low fiber volume fraction.
Another research path has been the use commercial closed cell foam technology to produce a lightweight rigid structure that expands to shape during processing. Unfortunately, all of the commercial foaming technology produces a gas that can become a contaminant as it diffuses through the polymer. A two component foaming technology is very difficult to control in terms of cell size and cell size distribution throughout a structure's cross-section.
With respect to boom structures utilized in space, boom structures currently are considered to fall into one of the five categories discussed below. The simplest boom structure is based on the use of tension members, typically wire. These structures have limited application, as the spacecraft must be spinning. Centrifugal force deploys and maintains the boom structure.
To date, probably the most common type of boom is the tubular boom. It consist of a thin metallic strip which is spool-wound during storage, but upon controlled release is automatically formed into a tubular shape giving it its stiffness. The tubular boom is, however, limited by its buckling strength at the root.
Another type of boom is the telescoping boom. A telescoping boom is typically shorter but has greater strength than a tubular boom. A telescoping boom is typically deployed using a lead screw or similar type device.
A fourth type of boom employs a series of continuous longerons, which are bent and twisted into a helix for stowage in a cylindrical container. Batten frames are arranged perpendicular to the longerons to provide the required separation, while diagonal cables are used to provide the required shear stiffness. This technology is limited by the amount of strain that can be placed on the longerons in the storage container. Booms of this type have been built in excess of 100 feet long.
The final boom type is similar to the one previously described except it uses articulated longerons instead of continuous longerons. In this type of structure, the longerons are in segments and connected with hinged joints to the batten frames. This approach also utilizes diagonal cables to provide the required shear stiffness.